1. Field of the Invention
The present invention relates to a friction engaging device such as a wet type multi-disc clutch etc incorporated into an automatic transmission for an automobile.
2. Related Background Art
Generally, an automatic transmission for an automobile includes, in addition to a torque converter defined as a fluid coupling, a planetary gear transmission mechanism on the order of 3-5 speed ranges, wherein the speed is changed by properly fixing or releasing components (a sun gear and a planetary gear etc) of the planetary gear transmission mechanism by a clutch and a brake. The clutch and the brake built in the automatic transmission involve the use of, excluding some of band type brakes, a wet type multi-disc type in which friction plates and separator plates are alternately disposed. An operation pressure oil from a variable speed control hydraulic circuit is used for fitting the two kinds of plates by pressure. Note that the wet type multi-disc clutch is used as a clutch for disconnecting the power in a motorcycle because of being capable of obtaining a comparatively large torque capacity with a small volume and being easy to make a smooth connection.
FIG. 20 is a vertical sectional view showing principal components of a conventional wet type multi-disc clutch by way of one example thereof. As illustrated in FIG. 20, a wet type multi-disc clutch 1 includes a cylindrical clutch case 2 connected to a certain component of a planetary gear transmission mechanism, a hub 3 connected to another component of the planetary gear transmission mechanism, and a piston 83 internally slidably fitted to a cylinder 81 provided on a proximal side (leftward in FIG. 20) of the clutch case 2. The clutch case 2 encases a plurality of separator plates 13 (four plates in an example shown in FIG. 20) and one single backing plate 19 which are so held as to be slidable in the axial direction, with a cylindrical retainer 85 fixed to an internal surface of the clutch case 2 being interposed therebetween. On the other hand, a plurality of friction plates 17 (four plates in the example shown therein) are so held as to be slidable in the axial direction on the hub 3 in the form of being interposed between the separator plates 13 and between the backing plate 19 and a separator plate 13. Referring again to FIG. 20, the numeral 87 designates an O-ring for sealing between the cylinder 81 and the piston 83, and 89 represents an oil chamber. In the wet type multi-disc clutch 1, when an operation pressure oil from an unillustrated variable speed control hydraulic circuit is introduced into the oil chamber 89, the piston 83 moves rightward in FIG. 20 at a predetermined velocity within the cylinder 81, and the separator plates 13 and the friction plates 17 are pressed by the piston, then press-fitted to each other and thereby frictionally engage with each other.
The prior art wet type multi-disc clutch described above inevitably has a variety of following problems which might arise when attempting to increase a friction engaging area and a pressure of the operation oil in order to ensure a large transfer torque capacity. For example, a method generally implemented for increasing the friction engaging area, is to increase the numbers and diameters of the friction plates 17 and the separator plates 13. If this method is taken, however, there increase the number of components and both of an axial dimension and outside diameter of the clutch case 2, resulting in a difficulty of being incorporated into the transmission. In addition, a weight of the device is bound to considerably increase. Further, though there is a method of enhancing a friction engaging force by raising a pressure of the operation pressure oil while using the same wet type multi-disc clutch, it is required in this case that a discharge pressure of a hydraulic pump be, as a matter of course, increased. Therefore, a high-pressure type hydraulic pump is needed on one hand, and the power dissipated by driving the hydraulic pump also increase on the other hand. If used as a component of the automatic transmission for the automobile, a travelling performance and a fuel consumption of the automobile might decline.
Such being the case, a variety of wet type multi-disc clutches contrived to increase the friction engaging force by use of a cam mechanism have been proposed for obviating the problems given above. What is typical thereof may be a clutch device disclosed in Japanese Patent Application Laid-Open Publication No. 7-259885 (which is hereinafter referred to as prior art 1), and a clutch device disclosed in Japanese Patent Application Laid-Open Publication No. 7-279992 (which is hereinafter referred to as prior art 2). As pointed out by Japanese Patent Application Laid-Open Publication No. 10-89381 (which is hereinafter referred to as prior art 3), however, according to the clutch devices based on the prior arts 1 and 2, once the friction engagement is made by the cam, the cam is firmly fitted in as if driving a wedge, and hence the friction engagement of the clutch can not be released even by depressurizing the operation oil. Namely, in those clutch devices, the clutch is not released till a driving torque in a reverse direction is applied, so that there arises a problem in which a controllability is extremely poor because of being incapable of connecting and disconnecting the clutch only by operating the oil pressure.
Further, the clutch devices according to the prior arts 1 and 2 incorporate a so-called one-way clutch function by which the clutch is automatically released upon a reverse of the direction of the driving torque. Based on this one-way clutch function, however, there is likewise a problem in which the cam is firmly fitted in when the friction engagement is done, then a reverse torque occurs in a driving system the instant the clutch is released, and a variable speed shock due to the occurrence of the reverse torque is transferred to the car body from the automatic transmission. A further defect is that the operation oil pressure is required to be released beforehand for working the one-way clutch function, and switching control of the oil pressure becomes intricate.
Moreover, the clutch devices according to the prior arts 1 and 2, though capable of effecting the automatic release corresponding to the direction of the transfer torque, do not have an automatic connecting function. In the automatic transmission adopting those clutch devices having no such function, when shifted up, a low-gear side clutch is automatically released by connecting a high-gear side clutch, thus completing the gear change. When shifted down, however, it is required that the low-gear side clutch be connected synchronizing with the release of the high-gear side clutch, the gear change must be done based on a clutch-to-clutch method.
On the other hand, the clutch device according to the prior art 3 aims at improving inconvenience caused by the reverse torque described above. However, as disclosed in the specification thereof, the occurrence of the reverse torque, though relieved, could not be completely obviated.
Furthermore, another typical example of the wet type multi-disc clutch using the cam mechanism may be a clutch device disclosed in Japanese Patent Application Laid Open Publication No. 3-219123 (which is hereinafter referred to as prior art 4). In this clutch device, however, in terms of using an electromagnetic clutch for a pilot clutch, there exists a necessity for separately using a magnetic body and a non-magnetic body as materials of the components thereof. In addition, a contrivance for keeping an air gap at a high accuracy is needed, which leads to a problem of largely increasing costs for the device. Incidentally, this clutch device, unlike the clutch devices according to the prior arts 1-3, does not incorporate the one-way clutch function.